This invention relates to nuclear reactors and to burnable neutron absorbers for such reactors. This invention relates particularly to neutron-absorbers such as are disclosed in Orr application. Orr discloses a neutron-absorber assembly including annular neutron-absorber pellets in a closed annular chamber or cavity between coaxial cylinders of Zircaloy alloy. The pellets are composed of ceramics including a matrix of a highly refractory material in which is embedded or encapsulated a burnable poison or burnable neutron-absorber. Typical matrix material are aluminum oxide, Al.sub.2 O.sub.3, and zirconium oxide, ZrO.sub.2. Certain isotopes of the elements boron, gadolinium, samarium, cadmium, europium, hafnium, dysprosium, and indium are burnable neutron absorbers. One or more of these elements in their natural state or enriched in the neutron-absorbing isotopes are encapsulated in the matrix.
As used in this application, the expression "neutron-absorber" means the material which captures neutrons; the expression "neutron-absorber body" means the body including the neutron-absorber in a matrix, also sometimes referred to as a "ceramic", and includes within its meaning a "pellet" as well as the body from which pellets are cut; the expression "neutron-absorber assembly" or "neutron-absorber rod" means the apparatus including the chambers with the pellets therein.
Usually the neutron-absorber, natural or enriched, is encapsulated as a compound. Of particular interest is boron whose isotope boron 10 is a neutron absorber. Typical ceramics are a matrix of Al.sub.2 O.sub.3 encapsulating B.sub.4 C or a matrix ZrO.sub.2 encapsulating zirconium boride, ZrB.sub.2. B.sub.4 C with depleted boron also may serve as a material for a burnable poison. The Al.sub.2 O.sub.3 +B.sub.4 C ceramic and the ZrO.sub.2 and ZrB.sub.2 ceramic may include natural boron or boron enriched or depleted in B.sup.10 with the quantity B.sup.10 varied depending on the radial wall thickness and density of the pellets and the purpose which it is to serve. In the B.sub.4 C with the depleted B.sup.10, the B.sup.10 is set to yield the required B.sup.10 loading per foot of the pellet. The primary neutron-absorber body includes a matrix of Al.sub.2 O.sub.3 encapsulating B.sub.4 C.
Since Al.sub.2 O.sub.3 and B.sub.4 C are most commonly used as matrix and neutron absorber, the text of this application deals predominantly with these constituents. It is to be understood that it is not intended that the invention should be confined to these compounds. To the extent that this invention is practiced with other compounds, either for the matrix or for the burnable absorber, such practice is regarded as within the scope of equivalents of the invention under the doctrine of equivalents as this doctrine is defined and explained by the U.S. Supreme Court in Graver Tank & Mfg. Co., Inc. et al. v. Linde Air Products Co. 339 U.S. 605; 70 S. Ct. 1017 (1950).
It has been proposed that the annular pellets be produced by forming a green body of powders of the matrix and the neutron absorber, sintering the green body and cutting and machining or grinding the resultign body to size. Typically the finished pellet so produced is about 2 inches in length. In the interest of economy, particularly to avoid excessive scrap in the finishing of individual pellets, and in the interest of practicability, a green body is produced which after sintering may be severed into blanks for forming several neutron-absorber bodies or pellets. Typically the cylinder is a tube 7 or 8 inches in length. About 2 to 3 finished pellets are derived from a 7 or 8 inch tube. The practice has been to grind the inside and outside of the sintered pellets. This is a costly and time-consuming operation. It is desirable that the grinding step be dispensed with.
Structurally the annular pellets are subject to critical demands. Annular pellets of very small radial dimension; i.e. radial thickness, and of very tight tolerances are required. The thickness (radially) is typically between 0.020 and 0.040 inches. The typical spacing between the outer diameter of the inner cylinder of Zircaloy alloy and the inner diameter of the outer cylinder is relatively small. It is then necessary that the dimensions of the pellet, particularly its radial thickness, shall be maintained within tight limits. Because precise B.sup.10 loading is essential to reactor operation, the density and wall thickness of the pellets are critical. In the operation of a reactor the nuclear reaction between the neutrons which bombard the B.sup.10 results in the formation of helium. In addition the bombardment of the pellets by neutrons displaces the atoms of the ceramic causing it to swell. The ceramics of which the pellets are formed are then porous, typically between about 60 and 80 percent of theoretical density. The requirement that the density be microscopically uniform applies to the percent theoretical density. The swelling and the emission of helium subjects the pellets to substantial pressure. It is then required that the pellets have substantial strength so that they can withstand the pressure. It is essential that this requirement be met since the pellets have small radial thickness. Not only must these properties of the pellets be uniform but they must also be reproducible from pellet to pellet of any batch of material for producing the pellets, and from batch to batch. These demands are applicable to the relatively long cylinder from which the pellets are formed. It is necessary that a long cylinder having a truly linear axis about which it is symmetric be formed and that this cylinder have uniform densityand wall thickness throughout. Such demands call for sophisticated manufacturing practice.
It is an object of this invention to provide a method for producing economically, burnable neutron-absorber bodies or pellets meeting the above demands. It is contemplated that this object will be met by forming a green body from the products of the components and sintering the green body to size, thus dispensing, to the extent practicable, with a final grinding.